A Novel Type of Glial Cell Associated with Nodes of Ranvier in Rat Optic Nerve

Differential Expression of Apolipoprotein D in Human Astroglial and Oligodendroglial Cells

Apolipoprotein D (Apo D) is a secreted lipocalin in the nervous system that may be related to processes of reinnervation and regeneration. Under normal conditions, Apo D is present in the central nervous system in oligodendrocytes, astrocytes, and some scattered neurons. To elucidate the regional and cellular distribution of Apo D in normal human brain, we performed double immunohistochemistry for glial fibrillary acidic protein (GFAP) and Apo D in samples of postmortem human cerebral and cerebellar cortices. Most of the GFAP-positive cells in the gray matter had features of protoplasmic astrocytes and were mainly Apo D-positive. Apo D staining was mostly confined to the cell soma and proximal processes, whereas GFAP extended to a rich and extensive array of processes. The fibrous astrocytes in the white matter were immunoreactive for GFAP but not for Apo D. In the white matter, Apo D was mainly detected in oligodendrocytes and extracellularly in the neuropil. The results of the present study support a specific behavior for each astrocyte type. These findings suggest that Apo D expression may be cell-specific, depending on the particular tissue physiology at the time of examination.

The emerging role of stem cells in ocular neurodegeneration: hype or hope?

Affecting over a hundred million individuals worldwide, retinal diseases are among the leading causes of irreversible visual impairment and blindness. Thus, an appropriate study models, especially animal models, are essential to furthering our understanding of the etiology, pathology, and progression of these diseases. In this review, we provide an overview of retinal disorders in the context of biotherapeutic approaches in these disorders.

Endothelin, astrocytes and glaucoma

It has become increasingly clear that astrocytes may play an important role in the genesis of glaucoma. Astrogliosis occurs in response to ocular stress or the presence of noxious stimuli. Agents that appear to stimulate reactive gliosis are becoming increasingly clear. One class of agents that is emerging is the endothelins (ETs; specifically, ET-1). In this review we examine the interactions of ET-1 with astrocytes and provide examples where ET-1 appears to contribute to activation of astrocytes and play a role in the neurodegenerative effects that accompany such reactivation resulting in astrogliosis. These actions are presented in the context of glaucoma although information is also presented with respect to ET-1's role in the central nervous system and brain. While much has been learned with respect to ET-1/astrocyte interactions, there are still a number of questions concerning the potential therapeutic implications of these findings. Hopefully this review will stimulate others to examine this potential.

Inhibition of reactive gliosis attenuates excitotoxicity-mediated death of retinal ganglion cells

Reactive gliosis is a hallmark of many retinal neurodegenerative conditions, including glaucoma. Although a majority of studies to date have concentrated on reactive gliosis in the optic nerve head, very few studies have been initiated to investigate the role of reactive gliosis in the retina. We have previously shown that reactive glial cells synthesize elevated levels of proteases, and these proteases, in turn, promote the death of retinal ganglion cells (RGCs). In this investigation, we have used two glial toxins to inhibit reactive gliosis and have evaluated their effect on protease-mediated death of RGCs. Kainic acid was injected into the vitreous humor of C57BL/6 mice to induce reactive gliosis and death of RGCs. C57BL/6 mice were also treated with glial toxins, alpha-aminoadipic acid (AAA) or Neurostatin, along with KA. Reactive gliosis was assessed by immunostaining of retinal cross sections and retinal flat-mounts with glial fibrillary acidic protein (GFAP) and vimentin antibodies. Apoptotic cell death was assessed by TUNEL assays. Loss of RGCs was determined by immunostaining of flat-mounted retinas with Brn3a antibodies. Proteolytic activities of matrix metalloproteinase-9 (MMP-9), tissue plasminogen activator (tPA), and urokinase plasminogen activator (uPA) were assessed by zymography assays. GFAP-immunoreactivity indicated that KA induced reactive gliosis in both retinal astrocytes and in Muller cells. AAA alone or in combination with KA decreased GFAP and vimentin-immunoreactivity in Mϋller cells, but not in astrocytes. In addition AAA failed to decrease KA-mediated protease levels and apoptotic death of RGCs. In contrast, Neurostatin either alone or in combination with KA, decreased reactive gliosis in both astrocytes and Mϋller cells. Furthermore, Neurostatin decreased protease levels and prevented apoptotic death of RGCs. Our findings, for the first time, indicate that inhibition of reactive gliosis decreases protease levels in the retina, prevents apoptotic death of retinal neurons, and provides substantial neuroprotection.

Determinants of regional and local diversity within the astroglial lineage of the normal central nervous system

Astrocytes are a major component of the resident non-neuronal glial cell population of the CNS. They are ubiquitously distributed throughout the brain and spinal cord, where they were initially thought to function in both structural and homeostatic capacities, providing the framework and environment in which neurons performed their parenchymal duties. However, this stroma-like view of astrocytes is no longer satisfactory. Mounting evidence particularly within the last decade indicates that astrocytes do not simply support neuronal activity but directly contribute to it. Congruent with this evolving view of astrocyte function in information processing is the emergent notion that these glial cells are not a homogeneous population of cells. Thus, astrocytes in various anatomically distinct regions of the normal CNS possess unique phenotypic characteristics that may directly influence the particular neuronal activities that define these regions. Remarkably, regional populations of astrocytes appear to exhibit local heterogeneity as well. Many phenotypic traits of the astrocyte lineage are responsive to local environmental cues (i.e., are adaptable), suggesting that plasticity contributes to this diversity. However, compelling evidence suggests that astrocytes arise from multiple distinct progenitor pools in the developing CNS, raising the intriguing possibility that some astrocyte heterogeneity may result from intrinsic differences between these progenitors. The purpose of this review is to explore the evidence for and mechanistic determinants of regional and local astrocyte diversity.

Control of division and differentiation in oligodendrocyte-type-2 astrocyte progenitor cells

Oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells give rise to oligodendrocytes and type-2 astrocytes in cultures of rat optic nerve. These progenitors are one of the few cell types in which most aspects of proliferation and differentiation can be manipulated in a defined in vitro environment. When exposed to platelet-derived growth factor (PDGF), O-2A progenitors divide a limited number of times before clonally related cells differentiate into oligodendrocytes with a timing similar to that seen in vivo. In contrast, O-2A progenitors grown in the absence of mitogen do not divide but differentiate prematurely into oligodendrocytes, and progenitors exposed to appropriate inducing factors differentiate into type-2 astrocytes. O-2A progenitors can become immortalized through at least two different mechanisms. First, when O-2A progenitors are exposed to a combination of PDGF and basic fibroblast growth factor (bFGF) these cells undergo continuous self-renewal in the absence of differentiation. In contrast, the application of bFGF alone is associated with premature oligodendrocytic differentiation of dividing O-2A lineage cells. Thus, cooperation between growth factors can modulate O-2A progenitor self-renewal in a defined chemical environment by eliciting a novel programme of division and differentiation which cannot be predicted from the effects of either factor examined in isolation. A further mechanism which allows prolonged self-renewal in the O-2A lineage is the generation of a stem cell. O-2A progenitors isolated from optic nerves of perinatal rats also have the capacity to give rise to a population of cells called O-2Aadult progenitors, which differ from their perinatal counterparts in many characteristics. Most importantly, O-2Aadult progenitors have a slow cell cycle, divide and differentiate asymmetrically and appear to have the capacity for prolonged self-renewal. Thus, immortalization in this lineage can also be achieved by the generation of a cell with stem cell-like characteristics from a rapidly dividing progenitor population.

Glial membranes at the node of Ranvier prevent neurite outgrowth

Nodes of Ranvier are regularly placed, nonmyelinated axon segments along myelinated nerves. Here we show that nodal membranes isolated from the central nervous system (CNS) of mammals restricted neurite outgrowth of cultured neurons. Proteomic analysis of these membranes revealed several inhibitors of neurite outgrowth, including the oligodendrocyte myelin glycoprotein (OMgp). In rat spinal cord, OMgp was not localized to compact myelin, as previously thought, but to oligodendroglia-like cells, whose processes converge to form a ring that completely encircles the nodes. In OMgp-null mice, CNS nodes were abnormally wide and collateral sprouting was observed. Nodal ensheathment in the CNS may stabilize the node and prevent axonal sprouting.

Role of Na(+)-Ca(2+) exchanger after traumatic or hypoxic/ischemic injury to spinal cord white matter

BACKGROUND CONTEXT

Spinal cord injury is a devastating condition in which clinical disability results from demyelination of white matter tracts. Changes in glial-axonal signaling, and enhanced Ca(2+) channel activity with excessive accumulation of intracellular Ca(2+), is a common phenomenon after hypoxia/ischemia or mechanical trauma to spinal cord dorsal column white matter tracts leading to irreversible injury.

PURPOSE

In the present study we examined the role of Na(+)-Ca(2+) exchanger (NCX) at physiological temperatures after hypoxia/ischemia and compressive injury to spinal cord dorsal column white matter in vitro.

STUDY DESIGN

A 30-mm length of dorsal column was isolated from the spinal cord of adult rats, pinned in an in vitro recording chamber (maintained at 37 degrees C) and injured by exposure to a hypoxic atmosphere for 60 minutes or compressed with a modified aneurysm clip (2-gm closing force) for 15 seconds. The functional integrity of the dorsal column was monitored electrophysiologically by quantitatively measuring the compound action potential (CAP) with glass microelectrodes.

RESULTS

The mean CAP decreased to 49.5 +/- 5.7% and 49.4 +/- 2.6% of control (p<.05) after hypoxia/ischemia and compressive injury, respectively. KB-R7943, a potent, selective NCX reverse mode inhibitor, significantly promoted greater recovery of CAP amplitude to 82.0 +/- 10.0% and 70.8 +/- 10.7% of control (p<.05) after hypoxic/ischemic or compressive injury to dorsal column white matter, respectively, when applied at 10 microM concentration. Bepridil (Research Biochemical Inc., Natick, MA, USA) (a less selective NCX inhibitor), when applied at 10 microM and 50 microM concentration promoted CAP amplitude recovery only to 46.8 +/- 7.8% and 29.9 +/- 3.3% of control, respectively, after hypoxic/ischemic injury to dorsal column white matter. Western blot analysis identified NCX presence with positive immunolabeling of 160 kD and 120 kD NCX proteins in the spinal cord white matter.

CONCLUSION

In conclusion, at physiological temperature NCX activation plays an important role in intracellular calcium overload after hypoxic/ischemic and compressive injury to spinal cord dorsal column white matter in vitro.

Thyrotropin-releasing hormone and its receptor in glia

The presence of thyrotropin-releasing hormone (Thyroliberin, TRH) and its receptor (TRH-R) in frozen coronal sections of the adult rat spinal cord and neonatal rat astroglial cultures was investigated by means of immunocytochemistry and Western blot using polyclonal antibodies generated against the hormone and monoclonal antibodies originated against discrete sequences of the type 1 rat TRH receptor (TRH-R1). TRH-R1 and TRH are present both in astroglial cells from adult rats and in cultured cells from newborn animals. The localization of TRH and TRH-R1 in nonneuronal cells in the central nervous system may reflect that some of the neurotrophic actions of TRH upon the central nervous system are mediated by glial cells.

Long-term culture of purified postnatal oligodendrocyte precursor cells. Evidence for an intrinsic maturation program that plays out over months

Oligodendrocytes myelinate axons in the vertebrate central nervous system (CNS). They develop from precursor cells (OPCs), some of which persist in the adult CNS. Adult OPCs differ in many of their properties from OPCs in the developing CNS. In this study we have purified OPCs from postnatal rat optic nerve and cultured them in serum-free medium containing platelet-derived growth factor (PDGF), the main mitogen for OPCs, but in the absence of thyroid hormone in order to inhibit their differentiation into oligodendrocytes. We find that many of the cells continue to proliferate for more than a year and progressively acquire a number of the characteristics of OPCs isolated from adult optic nerve. These findings suggest that OPCs have an intrinsic maturation program that progressively changes the cell's phenotype over many months. When we culture the postnatal OPCs in the same conditions but with the addition of basic fibroblast growth factor (bFGF), the cells acquire these mature characteristics much more slowly, suggesting that the combination of bFGF and PDGF, previously shown to inhibit OPC differentiation, also inhibits OPC maturation. The challenge now is to determine the molecular basis of such a protracted maturation program and how the program is restrained by bFGF.

Homotypic cell contact-dependent inhibition of astrocyte proliferation

The normal adult vertebrate nervous system is a relative quiescent tissue in terms of cell proliferation. However, astrocytes in many regions of the central nervous system (CNS) retain the capacity to undergo cell division. To examine the mechanisms that regulate the proliferation of astrocytes in the CNS we have utilized an in vitro assay in which astrocyte density and cellular environment could be regulated. We demonstrate that type 1 astrocytes derived from the cerebral cortex of developing rats exhibit a profound density-dependent inhibition of proliferation. This inhibition of proliferation was cell type specific, but not restricted to type 1 astrocytes. NIH 3T3 cells but not smooth muscle cells inhibited astrocyte proliferation, while contact-inhibited astrocytes did not inhibit oligodendrocyte proliferation. Co-culture of type 1 astrocytes with neurons from a variety of sources resulted in induction of a process-bearing astrocyte morphology and promoted glial cell proliferation. Thus, induction of a process-bearing astrocyte morphology does not lead to a cessation of proliferation. The inhibition of astrocyte proliferation did not appear to be mediated through the release or sequestration of soluble factors but rather could be induced by membrane-associated factors.

Immunolocalization of the Na(+)-Ca2+ exchanger in mammalian myelinated axons

Previous studies on the pathophysiology of white matter anoxic injury have revealed that the Na(+)-Ca2+ exchanger is an important mediator of Ca2+ overload. To date, however, the localization of this key Ca2+ transporter in myelinated axons has not been demonstrated. The present study uses immunofluorescence labeling with a monoclonal antibody (R3F1) to the canine cardiac type I Na(+)-Ca2+ exchanger to localize exchanger protein to rat peripheral and central myelinated axons. The indirect immunofluorescence labeling technique was used to study paraformaldehyde fixed frozen cryostat sections of sciatic nerve, optic nerve and spinal cord. Examination of sciatic nerve sections with both conventional and confocal microscopy revealed a staining pattern which suggested both a glial and axonal localization of the exchanger. In the rat optic nerve, positive label was associated with cell bodies and their processes, likely glia, and with numerous finer processes arranged in parallel, running longitudinally. These finer processes likely represent axonal profiles. A similar staining pattern was observed in lateral and dorsal columns from spinal cord. Immunoelectron microscopy of dorsal root axons revealed gold particles associated with the paranodal and internodal myelin, in the axoplasm, and close to the nodal/paranodal axon membrane. The high density of Na(+)-Ca2+ exchanger demonstrated in central and peripheral myelinated mammalian axons supports the importance of this transporter in Ca2+ regulation in these tissues.

Fate of developing astrocytes in the optic nerve of the myelin-deficient rat

There is considerable debate on the development of a glial cell line in the rat optic nerve, which is characterized by the specific expression of the A2B5 and HNK-1 epitopes. This cell line has been assumed to give rise to oligodendrocytes and so-called type 2 astrocytes. However, it is doubtful that the latter cell type really exists in vivo. In the present study, we have addressed this question by investigating the development of astrocytes in the myelin-deficient (md) rat, which is characterized by dysmyelination and loss of oligodendrocytes. Defective oligodendrocytes were observed by the third postnatal day, well before the generation of type 2 astrocytes. Consequently, the number of type 2 astrocytes was reduced in cultures prepared from optic nerves of md rats vs. controls. This finding was not paralleled in vivo; i.e., no dying astrocytes were observed in md sections by conventional electron microscopy. However, immunoreactivity against the HNK-1 epitope was enhanced in md compared to control sections. Ultrastructurally, HNK-1 immunoreactivity was detected predominantly on the axonal surface at astroaxonal contact sites, which were found only at the nodes of Ranvier within controls but extended to the whole axonal surface in md animals. Only a minor portion of the immunoreactivity derived from glial cells, presumably from oligodendrocytes at the paranodal region in controls. Thus, the HNK-1 epitope is not a useful antigen for distinguishing astrocytes in the rat optic nerve. Accordingly, our results do not provide evidence for the existence of specialized type 2 astrocytes in vivo. In vitro, these cells are probably only oligodendrocytes that mimic some astroglial features if grown in serum-containing media.

Glial lineages and myelination in the central nervous system

Oligodendrocytes, derived from stem cell precursors which arise in subventricular zones of the developing central nervous system, have as their specialist role the synthesis and maintenance of myelin. Astrocytes contribute to the cellular architecture of the central nervous system and act as a source of growth factors and cytokines; microglia are bone-marrow derived macrophages which function as primary immunocompetent cells in the central nervous system. Myelination depends on the establishment of stable relationships between each differentiated oligodendrocyte and short segments of several neighbouring axons. There is growing evidence, especially from studies of glial cell implantation, that oligodendrocyte precursors persist in the adult nervous system and provide a limited capacity for the restoration of structure and function in myelinated pathways damaged by injury or disease.

Immunohistochemical study of human optic nerve head astroglia

Immunocytochemical localization of glial fibrillary acidic protein (GFAP) has been used to study the distribution of astrocytes and their morphology in sections of the optic nerve (ON) of human eye. Although all ON regions presented GFAP immunoreactivity, immunostained tissue was most common in the posterior prelaminar region (PR) and least common in the laminar region (LR). Two shapes of astrocytes were distinguished: thick and thin bodied astrocytes. Astrocytes with thick cell bodies are located in the superficial nerve fiber layer (SNFL), PR, LR and retrolaminar region (RR). Astrocytes with thin cell bodies were found in the SNFL and anterior PR. Sometimes thin bodied astrocytes presented another shape with a long process running parallel to the axons and these were found in the PR and LR. In the SNFL the thin bodied astrocytes accompany the axons and contact the capillaries derived from the central retinal artery. In the anterior PR the thin bodied astrocytes with a stellate shape lie over the vessels forming a sieve through which the axons pass. In the posterior PR, the thick bodied astrocytes form glial tubes that direct axons towards the LR. These astrocytes form a layer in the LR that lines the pores of the lamina cribrosa and separates the connective septa from the axon bundles in the RR. The limiting glial membranes separate the ON tissues from the adjacent tissues and from the course of the central retinal artery and are composed of many thick bodied astrocytes.

Glial cells in transected optic nerves of immature rats. I. An analysis of individual cells by intracellular dye-injection

The glial response to Wallerian degeneration was studied in optic nerves following unilateral enucleation in immature rats, aged 21 days old (P21). The three-dimensional morphology of dye-filled glia was determined in intact nerves, at post-enucleation day 21 in normal nerves from untreated P21 rats, by correlating laser scanning confocal microscopy and camera lucida drawings of single cells. In normal and transected nerves, the majority of dye-filled cells comprized astrocytes (54% and 65%, respectively). In normal P21 nerves, the predominant astrocyte form had a complex stellate morphology and had a centrally-located cell body from which branching processes extended randomly. Two other distinct forms were transverse and longitudinal astrocytes, which had a polarized process extension in a plane perpendicular or parallel to the long axis of the nerve, respectively. These forms were recognized in transected nerves also, but astrocytes in transected nerves had a simple morphology on the whole, and extended few, dense processes which branched infrequently. Quantitative analysis of astrocyte morphology confirmed that individual astrocytes underwent considerable remodelling in response to Wallerian degeneration. A prominent reaction was that astrocytes had withdrawn radial processes and extended a greater proportion of processes longitudinally, parallel to the long axis of the nerve and along the course of degenerated axons. A further, notable feature of transected nerves was the development of novel longitudinal forms and of hypertrophic astroglia. These results indicated that all astrocytes became reactive following enucleation and that glial scar formation was not the function of a single astrocyte subtype. Oligodendrocytes in transected nerves had lost their myelin sheaths and appeared as small cells with numerous bifurcating processes which extended radially, but a small number of oligodendrocytes were recognized which apparently supported myelin sheaths (9%, compared to 40% in normal nerves). In addition, there was a significant population of indeterminate cells in transected nerves (26%, compared to 6% in normal nerves) and, although some of these were identified as microglia/macrophages, it was concluded that many were likely to be dedifferentiated oligodendrocytes.

Heterogeneous morphology and tracer coupling patterns of retinal oligodendrocytes

The present study characterizes the morphology and tracer coupling patterns of oligodendrocytes in the myelinated band of the rabbit retina, as revealed by intracellular injection of biocytin or Lucifer yellow in an isolated superfused preparation. Based on the observed heterogeneity in morphology, we have grouped the presumptive oligodendrocytes into three categories termed 'parallel', 'stratified' and 'radial'. Most parallel oligodendrocytes were tracer coupled to nearby oligodendrocytes and astrocytes, whereas the stratified and radial oligodendrocytes rarely showed coupling. We conclude that the different categories of oligodendrocytes may be stages in a developmental series, with radial oligodendrocytes being premyelinating cells, parallel oligodendrocytes being mature myelinating cells and the stratified cells representing a transition between these categories.

Strongly GD3+ cells in the developing and adult rat cerebellum belong to the microglial lineage rather than to the oligodendrocyte lineage

A recent study has shown that ramified microglia in the adult rat optic nerve express the ganglioside GD3 [Wolswijk Glia 10:244-249, 1994], thereby raising the possibility that some GD3+ in the developing rat central nervous system (CNS) belong to the microglial lineage rather than to the oligodendrocyte lineage, as previously thought. To examine this possibility, sections of postnatal and adult cerebellum were double-labelled with markers for rat microglia [the B4 isolectin derived from Griffonia simplicifolia (GSI-B4), the ED1 monoclonal antibody (mAb), and the OX-42 mAb] and anti-GD3 mAbs (the mAbs R24 and LB1). These immunolabellings showed that ramified microglia as well as amoeboid microglia are strongly GD3+ in vivo. Moreover, most, if not all, cells that express high levels of GD3 in sections of developing cerebellum appear to belong to the microglial lineage. These observations contradict previous suggestions that the strongly GD3+ cells in the putative white matter regions of the developing brain are oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells; the cells that give rise to oligodendrocytes in the CNS. The present study did, however, confirm that some O-2A progenitor cells in sections of postnatal cerebellum are weakly GD3+ in vivo. Amoeboid microglia are present in areas of the developing cerebellum where newly generated oligodendrocytes are found, suggesting that these cells play a role in the phagocytosis of the large numbers of oligodendrocytes that die as part of CNS development.

Glial differentiation: a review with implications for new directions in neuro-oncology

Major advances in cell culture techniques, immunology, and molecular biology during the last 10 years have led to significant progress in understanding the process of normal glial differentiation. This article summarizes our current understanding of the cellular and molecular basis of glial differentiation based on data obtained in cell culture and reviews current hypotheses regarding the transcriptional control of the gene switching that controls differentiation. Understanding normal glial differentiation has potentially far-reaching implications for developing new forms of treatment for patients with glial neoplasms. If oncogenesis truly involves a blockage or a short circuiting of the differentiation process in adult glial progenitor cells, or if it results from dedifferentiation of previously mature cells, then a clear understanding of differentiation may provide a key to understanding and potentially curtailing malignancy. Differentiation agents represent a relatively new class of drugs that effect cellular gene transcription at the nuclear level, probably through alterations in chromatin configuration and/or differential gene induction. These exciting new agents may provide a means of preventing the dedifferentiation of low-grade gliomas or inducing malignant glioma cells to differentiate with minimal toxicity. In the future, genetic therapy has the potential of more specifically rectifying the defect in genetic control that led to oncogenesis in any given tumor.

Astrocyte associations with nodes of Ranvier: ultrastructural analysis of HRP-filled astrocytes in the mouse optic nerve

Astrocytes are implicated in the function of nodes of Ranvier because their perinodal processes form contacts with the axonal membrane at nodes. We have filled astrocytes iontophoretically with horseradish peroxidase in the intact mouse optic nerve to resolve the precise relationship between perinodal processes and astrocyte three dimensional structure. We confirm that nodal contacts were formed either by single processes which almost completely enveloped nodes, or by delicate, finger-like projections from larger processes which made discrete nodal contacts. A single perinodal process can form multiple contacts with a node and nodes were contacted by processes from more than one astrocyte. Perinodal processes emanated from larger processes, which terminated as end-feet on blood vessels and at the pia, as well as collateral branches which subsequently ended at nodes; these latter may specifically subserve nodes. Perinodal contacts were also formed directly by the soma and cytoplasmic expansions of the cell body. Both primary processes and collateral branches formed multiple associations with nodes which often appeared in clusters. Thus, all astrocytes formed multiple contacts with nodes, blood vessels and the subpial glia limitans. We conclude that perinodal processes are not formed by a specialized astrocyte in the mouse optic nerve.

Three-dimensional morphology of astrocytes and oligodendrocytes in the intact mouse optic nerve

The three-dimensional morphology of astrocytes and oligodendrocytes was analysed in the isolated intact mature mouse optic nerve, by correlating laser scanning confocal microscopy and camera lucida drawings of single cells, dye-filled with lysinated rhodamine dextran or horseradish peroxidase, respectively. These techniques enabled the entire process field of single dye-filled cells to be visualized in all planes and resolved the fine details of glial morphology. Morphometric analysis showed that the processes of all astrocytes had branches ending at the pial surface, on blood vessels, and freely in the nerve; branches ending in the nerve were described to end at nodes of Ranvier in the accompanying paper. Astrocytes were classified into a single morphological population in which each cell subserved multiple functions. The results of this study do not support the contention that astrocytes can be subdivided into two morphological and functional subtypes, namely type-1 and type-2, which have process ending either at the glia limitans or at nodes, respectively. Three-dimensional analysis of oligodendrocyte units, defined as the oligodendrocyte, its processes and the axons it ensheaths, showed the provision of single myelin segments for an average of 19 nearby axons (range 12-35) with a mean internodal length of 138 microns (range 50-350 microns). Mouse optic nerve oligodendrocytes were a homogeneous population and were markedly similar to those in the rat optic nerve. The results of our analysis of oligodendrocyte morphology are consistent with the view that the number and internodal length of myelin sheaths supported by a single oligodendrocyte are related to the diameter of the ensheathed axons.

Confocal imaging of glial cells in the intact rat optic nerve

Astrocytes and oligodendrocytes in the isolated intact mature rat optic nerve have been computer imaged in three dimensions by laser scanning confocal microscopy of single cells, dye-filled with lysinated rhodamine dextran (LRD). Our results illustrate the first application of these techniques to an intact CNS white matter tract and provide comparative data for previous studies on neonatal rat optic nerve (Butt and Ransom: Glia 2:470-475, 1989; Butt and Ransom: J Comp Neurol 338:141-158, 1993). The combined use of intracellular injection of LRD and confocal imaging significantly improves the resolution of glial cell structure, particularly that of mature astrocytes, for a number of reasons. 1) Single mature dye-filled glia can be imaged, because LRD does not pass through gap junctions. 2) The entire process field of astrocytes can be visualized in a single two-dimensional image. 3) Cell images can be rotated through 360 degrees in all planes to provide a new perspective of glial cell structure in the intact tissue. 4) Reconstruction of optical sections, within a narrow focal plane, provides a high definition and resolution of the finer details of glial morphology. Using these techniques, three astrocyte subclasses were distinguished on morphological criteria. It is the conclusion of this study that the majority of these forms represent a single population of fibrous astrocytes which are well-suited to perform the multiple functions attributed to astrocytes in the CNS. The morphology of mature myelin-forming oligodendrocytes was also described.(ABSTRACT TRUNCATED AT 250 WORDS)

Future prospects for the management of multiple sclerosis

A comprehensive strategy for the future management of multiple sclerosis will involve limiting the inflammatory process and repairing the damage. Monoclonal antibody therapy offers one means for achieving very rapid and substantial antiinflammatory effects, but the presently available reagents will almost certainly not prove to be definitive. Other candidates in the cascade of events that leads to myelin destruction will need to be considered, including TNF-alpha and other cytokines. But for the large number of individuals who are currently disabled, the more ambitious but realizable approach of glial repair holds the best hope for reversing persistent disabilities, transferring the technology and discoveries of contemporary experimental cellular neurobiology to the clinic.

Morphology of astrocytes and oligodendrocytes during development in the intact rat optic nerve

The detailed three-dimensional morphology of macroglial cells was determined throughout postnatal development in the intact rat optic nerve, a central nervous system white matter tract. Over 750 cells were analyzed by intracellular injection of horseradish peroxidase or Lucifer Yellow to provide a new perspective of glial differentiation in situ. Retrograde analysis of changes in glial morphology allowed us to identify developmental timetables for three morphological subclasses of astrocytes and oligodendrocytes, and to estimate their time of emergence from undifferentiated glial progenitors. Glial progenitors were recognised throughout postnatal development and persisted in 35-day-old nerves, where we suggest they represent adult progenitor cells. Astrocytes were present at birth, but the majority of these cells developed over the first week as three morphological classes emerged having either transverse, random, or longitudinal process orientation. Several lines of evidence led us to believe that the majority of astrocytes in the rat optic nerve were morphological variations of a single cell type. Young oligodendrocytes were first observed 2 days after birth, indicating that they diverged from progenitors at or near this time. During early development these cells extended a large number of fine processes, which then bifurcated and extended along axons. Later, as myelination proceeded, oligodendrocytes exhibited fewer processes which grew symmetrically and uniformly along the axons, resulting in a highly stereotypic mature oligodendrocyte form. Our analysis of oligodendrocyte growth suggests that these cells did not myelinate axons in a random manner and that axons may influence the myelinating processes of nearby oligodendrocytes.

Expression of janusin (J1-160/180) in the retina and optic nerve of the developing and adult mouse

We have analyzed the expression of the oligodendrocyte-derived extra-cellular matrix molecule janusin (previously termed J1-160/180) in the retina and optic nerve of developing and adult mice using indirect light and electron microscopic immunocytochemistry, immunoblot analysis, and enzyme-linked immunosorbent assay. In the optic nerve, janusin is not detectable in neonatal and only weakly detectable in 7-day-old animals. Expression is at a peak in 2- or 3-week-old animals and subsequently decreases with increasing age. In the retina, expression increases until the third postnatal week and then remains at a constant level. In immunocytochemical investigations at the light microscopic level, janusin was found in the myelinated regions of the nerve with spots of increased immunoreactivity possibly corresponding to an accumulation of the molecule at the nodes of Ranvier. At the electron microscopic level, contact sites between unmyelinated axons, between axons and glial cells, and between axons and processes of myelinating oligodendrocytes were immunoreactive. Cell surfaces of astrocytes at the periphery of the nerve and forming the glial-limiting membrane, in contrast, were only weakly immunopositive or negative. In cell cultures of young postnatal mouse or rat optic nerves, oligodendrocytes and type-2 astrocytes, but not type-1 astrocytes were stained by janusin antibodies. In the oligodendrocyte-free retina, janusin was detectable in association with neuronal cell surfaces, but not with cell surfaces of Müller cells or retinal astrocytes. Our observations indicate that expression of janusin in the optic nerve and in the retina is developmentally differentially regulated and that other cell types, in addition to oligodendrocytes, express the molecule. Since the time course of janusin expression in the optic nerve coincides with the appearance of oligodendrocytes and myelin and since janusin is associated with cell surfaces of oligodendrocytes and outer aspects of myelin sheaths and is concentrated at nodes of Ranvier, we suggest that janusin is functionally involved in the process of myelination.

Glia cells of the monkey retina. I. Astrocytes

Morphology and distribution of retinal astrocytes have been studied in macaque monkeys by immunocytochemical localization of glial fibrillary acidic protein (GFAP). With the exception of the fovea and the far periphery, astrocytes are ubiquitous in the nerve fiber layer (NFL) and the ganglion cell layer (GCL) of the monkey retina. The morphology of NFL astrocytes changes gradually, from star-shaped in the periphery to bipolar close to the optic disc. By contrast, GCL astrocytes maintain their star-shaped appearance throughout the retina. Astrocytes are unevenly distributed in the monkey retina, showing the highest concentration around the optic disc, and particularly low densities in the perifoveal region and the far periphery. The fovea proper is devoid of astrocytes. Employing high-resolution confocal microscopy, we could demonstrate that astrocytes form manifold contacts to blood vessels. In addition, bundles of NFL astrocyte processes are co-localized with axon bundles, individual astrocytes forming contacts to several axon bundles. In contrast, a similar affinity of astrocytes to ganglion cell somata was never observed. Thus, our data confirm and extend the current knowledge of morphology and putative function of astrocytes in mammalian and especially the primate retina.

N- and R-cadherin expression in the optic nerve of the chicken embryo

Cadherins are a family of molecules mediating Ca(2+)-dependent cell-cell adhesion in various tissues. N- and R-cadherin are expressed in the chick embryonic CNS and differ in their expression pattern during development. Here we focus on the differential expression of N- and R-cadherin in the early optic nerve. N-cadherin is expressed by the retinal neurites growing through the optic nerve. R-cadherin is expressed by the early optic nerve glia, which derives from the optic stalk neuroepithelium and corresponds to an immature form of the type-1 astrocyte described in rat optic nerve. The close contact between the plasma membranes of the retinal neurites and the optic nerve glia is believed to be important in guiding retinal axons through the optic nerve. Using neuroblastoma cell lines transfected with R-cadherin, we demonstrate that the N-cadherin-positive retinal axons can use R-cadherin as a substrate for axon elongation. These results suggest that the R-cadherin expressed by the early optic nerve glia might provide a molecular substrate for the growth of N-cadherin-positive retinal axons through the optic nerve.

Inflammation and the brain

Inflammation in the brain selectively damages the myelin sheath resulting in a variety of clinical syndromes of which the most common is multiple sclerosis. In these disorders, the areas of inflammation and demyelination can be identified in life by magnetic resonance imaging. Events occurring at the blood-brain barrier depend on T-cell activation, which increases immune surveillance within the central nervous system. T-cells activated against brain antigens persist to establish the conditions needed for inflammatory demyelination and this depends on local release of cytokines, culminating in removal of oligodendrocytes and their myelin lamellae by macrophages or microglia. These interactions involve binding between receptors present on microglia for the Fc portion of antibody and complement components to corresponding ligands on target cells. Taken together, the evidence from clinical and experimental studies provides a rationale for the issue of immunological treatments in patients with multiple sclerosis.

Molecular profile of reactive astrocytes--implications for their role in neurologic disease

The central nervous system responds to diverse neurologic injuries with a vigorous activation of astrocytes. While this phenomenon is found in many different species, its function is obscure. Understanding the molecular profile characteristic of reactive astrocytes should help define their function. The purpose of this review is to provide a summary of molecules whose levels of expression differentiate activated from resting astrocytes and to use the molecular profile of reactive astrocytes as the basis for speculations on the functions of these cells. At present, reactive astrocytosis is defined primarily as an increase in the number and size of cells expressing glial fibrillary acidic protein. In vivo, this increase in glial fibrillary acidic protein-positive cells reflects predominantly phenotypic changes of resident astroglia rather than migration or proliferation of such cells. Upon activation, astrocytes upmodulate the expression of a large number of molecules. From this molecular profile it becomes apparent that reactive astrocytes may benefit the injured nervous system by participating in diverse biological processes. For example, upregulation of proteases and protease inhibitors could help remodel the extracellular matrix, regulate the concentration of different proteins in the neuropil and clear up debris from degenerating cells. Cytokines are key mediators of immunity and inflammation and could play a critical role in the regulation of the blood-central nervous system interface. Neurotrophic factors, transporter molecules and enzymes involved in the metabolism of excitotoxic amino acids or in the antioxidant pathway may help protect neurons and other brain cells by controlling neurotoxin levels and contributing to homeostasis within the central nervous system. Therefore, an impairment of astroglial performance has the potential to exacerbate neuronal dysfunction. Based on the synopsis of studies presented, a number of issues become apparent that deserve a more extensive analysis. Among them are the relative contribution of microglia and astrocytes to early wound repair, the characterization of astroglial subpopulations, the specificity of the astroglial response in different diseases as well as the analysis of reactive astrocytes with techniques that can resolve fast physiologic processes. Differences between reactive astrocytes in vivo and primary astrocytes in culture are discussed and underline the need for the development and exploitation of models that will allow the analysis of reactive astrocytes in the intact organism.

Localization of janusin mRNA in the central nervous system of the developing and adult mouse

Janusin (formerly termed J1-160/180) is an oligodendrocyte-derived extracellular matrix molecule which is restricted to the central nervous system and which is expressed late during development (Pesheva et al., J. Cell Biol., 1765-1778, 1989). To gain insights into the molecule's morphogenetic functions and to identify its cellular source in vivo, we have studied the localization of janusin messenger RNA in the optic nerve, retina and spinal cord and the expression of janusin protein in the spinal cord of developing and adult mice. Moreover, we have analysed optic nerve cell cultures and retinal cell suspensions in double-labelling experiments using a janusin-specific anti-sense complementary RNA probe and cell type-specific antibodies to identify the cell types containing janusin transcripts. In developing animals, oligodendrocytes were strongly labelled with the janusin anti-sense cRNA probe during the period of myelination. The number of labelled cells and intensity of the hybridization signal decreased significantly with increasing age. Interestingly, expression of janusin was not confined to oligodendrocytes. Some neuronal cell types and type-2 astrocytes present in optic nerve cell cultures also contained janusin transcripts. In contrast to oligodendrocytes, the number and labelling intensity of neurons containing janusin transcripts remained constant during postnatal development and into adulthood. Expression of janusin protein in the spinal cord was developmentally regulated, with a peak of expression in 2- or 3-week-old animals. The molecule was visible in the white and grey matter. In myelinated regions, it was associated with myelinated fibres and accumulated at nodes of Ranvier. These observations suggest that janusin may be of functional relevance for myelination.

Expression of gamma-aminobutyric acid immunoreactivity in reactive astrocytes after ischemia-induced injury in the adult forebrain

Transient ischemia induces an increase in glial fibrillary acidic protein (GFAP) immunoreactivity which can be detected in specific forebrain regions of the adult gerbil as early as day 2, becomes prominent by day 4-7 and persists for at least 3 months. These forebrain areas include layers 2/3 of the somatosensory and auditory cortices, the CA1 and CA4 sectors of the hippocampus, the dorsolateral region of the striatum, and the dorsolateral subregion of the medial septal nucleus. In addition, astrocytes in the ischemically lesioned areas stain with gamma aminobutyric acid (GABA) antiserum. These GABA-immunoreactive astrocytes are not found in non-damaged areas. The time-course of expression of GABA immunoreactivity is similar to that of GFAP immunoreactivity. Using a double immunofluorescent staining method, reactive astrocytes which express GABA immunoreactivity were also found to immunostain with either GFAP or vimentin. On the other hand, astrocytes were not found to be immunoreactive with antibodies to glutamic acid decarboxylase or glutamate. Our present finding demonstrates, in an in vivo model, an aberrant expression of GABA immunoreactivity by astrocytes which is not observed in non-ischemic adult animals.

What have tissue culture studies told us about the development of oligodendrocytes?

One major success of studying neural cell development in tissue culture has been the discovery of the O-2A cell. This bipotential cell generates oligodendrocytes or, under certain conditions, a type of astrocyte. This essay considers the evidence that the characteristic properties demonstrated by the O-2A cells in vitro are an accurate reflection of oligodendrocyte development in vivo.

Cellular organisation of the optic nerve and the implications for optic neuritis

Opportunities for studying growth, degeneration and repair in the central nervous system have altered over the last decade with the development of techniques for culturing neurones and glia and the availability of immunological or molecular markers that identify separate lineages and their progeny. Much pioneering work has been carried out in the rodent optic nerve but the principles that emerge are representative for other parts of the nervous system; development of neurones and glia may differ substantially in rats and man, so that assumptions must be made in extrapolating from properties of the rat optic nerve to diseases of the human central nervous system.

The glial framework of central white matter tracts: segmented rows of contiguous interfascicular oligodendrocytes and solitary astrocytes give rise to a continuous meshwork of transverse and longitudinal processes in the adult rat fimbria

The cellular skeleton of the adult rat fimbria consists of regularly spaced interfascicular glial rows of considerable length, running in the longitudinal (axonal) axis of the tract. Each row consists of a series of repeated segments made up of a stretch of interfascicular oligodendrocytes lying in direct contact with each other, and separated from the adjacent segments by usually solitary interfascicular astrocytes. A typical segment would be around 60 microns long, and have an axial core of about eight contiguous oligodendrocytes surrounded by a shell of about 1,200 axons, 70% of which are myelinated. In the transverse plane of the tract, adjacent segments are stacked together with a core-to-core distance of around 15 microns. The interfascicular oligodendrocytes have radial stem processes (in a plane transverse to the axonal axis) which give rise to the longitudinal myelinating (internodal) processes. Both transverse and longitudinal oligodendrocytic processes are longer than the dimensions of the segment (in which their cell bodies lie) and its axonal shell. They thus cooperate in myelinating axons of adjacent segments in both planes. The interfascicular astrocytes have three distinct types of processes: radial, longitudinal, and vascular (bearing end feet). The radial astrocytic processes are thick and tapering, and the processes of individual astrocytes extend transversely (in the plane of the original embryonic radial glial processes) for a total of at least 100 microns. The considerably more numerous longitudinal astrocytic processes arise from all parts of the cell bodies and radial processes. They are up to at least 30 microns long, thin, untapering, and largely unbranched, and are interdigitated among the fimbrial axons. In the radial plane, the astrocytic radial processes spread out through a wide swathe of adjacent segments, so that the integrated meshwork of interpenetrating longitudinal processes arising from overlapping radial processes of astrocytes from many different interfascicular rows provides a continuous longitudinal substrate for the fimbrial axons.

The CNS-PNS transitional zone of the rat. Morphometric studies at cranial and spinal levels

The transitional zone is that length of rootlet containing both central and peripheral nervous tissue. The CNS-PNS interface may be defined as the basal lamina covering the intricately interwoven layer of astrocyte processes which forms the CNS surface and which is pierced by axons passing between the CNS and PNS. Study of transitional zone development defines morphologically the growth, relative movement and interaction of central and peripheral nervous tissues as they establish their mutually exclusive territories on either side of the CNS-PNS boundary, and helps to explain the wide variations in the form of the mature transitional zone. Nerve rootlets at first consist of bundles of bare axons. These become segregated by matrices of fine Schwann cell processes peripherally and of astrocyte processes centrally. The latter may prevent Schwann cell invasion of the CNS. Astrocyte processes branch profusely and come to form the principal central nervous tissue component of the transitional zone. Developmental changes in the transitional zone vary markedly between nerves, reflecting differences in its final morphology. Widespread relative movements and migration of CNS and PNS tissues take place during development, so that the central-peripheral interface changes shape and position, commonly oscillating along the proximodistal axis of the rootlet. For example, developing cervical ventral rootlets contain a transient central tissue projection, while that of lumbar ventral rootlets and to a lesser extent that of cervical dorsal rootlets alternately increase and decrease in length. In the developing cochlear nerve, a central tissue projection is present before birth, but regresses somewhat before a marked outgrowth of central nervous tissue along the nerve takes place, which reaches into the modiolus during the first week postnatum. During development, some astrocytic tissue may even break off and migrate distally into the root, giving rise to one or more glial islands within it. During the period immediately preceding birth, Schwann cells come to be present in very large numbers in that part of the rootlet immediately distal to the CNS-PNS interface, the proximal rootlet segment. Here they form prominent sleeves or clusters of closely packed cells which intertwine with and encapsulate one another on the rootlet surface. Such Schwann cell overcrowding in the proximal rootlet segment could result in part from distal overgrowth of the rapidly expanding CNS around axon bundles, which might strip the Schwann cells distally off the bundle segments so engulfed.(ABSTRACT TRUNCATED AT 400 WORDS)

Macroglial cell types, lineage, and morphology in the CNS

The characterization of macroglial antigenic phenotypes has provided considerable information on the classification of glial subtypes and lineage, but the picture in vivo appears to more complex than originally surmised from in vitro systems. Whether type-2 astrocytes and O-2A progenitors play a significant part in vivo remains controversial, but these questions have generated a valuable reassessment of inferences made from in vitro studies. It is not known whether disparate regions of the nervous system have different glial cell types with distinct cell lineages, but results in peripheral nerve and the cortex (see Price and colleagues, and Jessen and colleagues, this volume) suggest that this is likely the case. The heterogeneity of astrocytic form and the implied divergence of function in vivo parallel but have yet to be equated with the wide range of antigenic and electrophysiologic characteristics that can be induced in glia under certain culture conditions in vitro. Whether similar conditions prevail within the developing nervous system is not clear. The continued development of immunocytochemical markers and the use of retroviral vectors, combined with morphologic and electrophysiologic studies in vivo and in vitro, will undoubtedly answer these and other questions.

Limiting and repairing the damage in multiple sclerosis

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Aureolus Philippus Theophrastus Bombastus Von Hohenheim or Paracelsus 1493-1541

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Three-dimensional fine structure of cytoskeletal-membrane interactions at nodes of Ranvier

Cytoskeleton-membrane-extracellular matrix interactions at the node of Ranvier were examined in both central and peripheral axons by combining three different methods for tissue preparation with three different electron microscopic techniques for imaging supramolecular structure. Conventional and three-dimensional high voltage electron microscopy of thin and semithick sections of tissues stained en bloc with ferric chloride revealed the presence of transcellular structures across the nodal gap traversing the paranodal glial-axonal junction. These structures penetrate both axonal and glial membranes and are further traced to the cortical axoplasm. This observation was verified by an examination of similar regions in rapidly-frozen freeze-substituted fresh axons. The filamentous nature of these structures, their focal attachment to the external true surface of the nodal and paranodal axolemma and their association with membrane particles were visualized in deep etch rotary-shadow replicas. At the node, both extracellular gap-crossing filaments and membrane-cytoskeletal linkers in the nodal axoplasm are joined to one of the prominent membrane particles of the nodal axolemma. At the paranodal axo-glial junction, the anchoring site of these membrane-cytoskeleton linkers are found on the linear arrays of 16 nm particles. Thus, cytoplasmic filaments and extracellular filaments or bridge structures are involved in the membrane-cytoskeletal interaction at the node and paranode. Some of these membrane particles are known to play a role in ionic conductances known to occur at this site. An additional role in cell adhesion or maintenance of the membrane specialization of this functionally important site of axolemma is now indicated.

Bellini, Carpaccio, and receptors in the central nervous system

With the convergence of science from the fields of neurobiology and immunology, many exciting and challenging surprises have emerged regarding cytokines, neuroendocrine hormones, neuropeptides, excitatory amino acids, and their receptors. For some time neurobiologists have known that subsets of neural cells had different receptors for the same ligand. Those subsets of cells could be as different as neurons and astrocytes and as closely related as astrocytes from different lineages or anatomical areas. The neurobiological puzzle has been to determine the functional meaning of these differences. Immunologists in contrast have long understood the clear cut differences between T and B lymphocytes or T helper/inducer and T cytotoxic/suppressor cells and their response to cytokines. However, it is only very recently that they have discovered preferential use by these cells of different receptors for an identical cytokine ligand. Indeed, identical cytokines in the central nervous system and immune response may induce their pleiotropic responses by utilizing different receptors in these two systems. Immunologic paradigms may help neurobiologists predict the existence of subsets of neural cells and their function. Likewise, neurobiology may enable immunologists to predict roles for receptors in gene families as well as the existence of as yet unidentified receptors.

The design of the optic nerve in fish

Fish have large eyes, with short optic nerves that are continually flexed by compensatory eye movements during swimming. Here, I review the tissue construction of the fish optic nerve, to see how the glia and axons are adapted to withstand these mechanical stresses, which are not normally encountered by CNS tissue within the skull. As in other lower vertebrates, the optic nerve astrocytes are highly unusual: their intermediate filaments are composed of cytokeratins (Giordano et al., 1989), not GFAP. Their processes are linked together by desmosomes, forming thin transverse lace-like partitions, placed at quasi-regular intervals longitudinally (Maggs & Scholes, 1990). This accordion-like arrangement is interpreted as providing a flexible tissue-skeleton for the optic nerve. A new observation is that the optic axons run in coherent parallel waves. This pattern, which is complementary to that of the astroglia, reversibly accommodates limited axial stretches. The waves are equivalent to those underlying the optical banding of Fontana (1781) in peripheral nerves, but wavelength (30 microns) and amplitude (5 microns) are about an order of magnitude less, reflecting the much smaller average size of the optic axons. The pattern also occurs in mammals, and may be restricted to the visual pathway: if present elsewhere in the CNS, nerve-fiber waves are inconspicuous at best. In fish, the astroglial partitions occur in register with the waves, suggesting that steric interactions between developing axons and glia may help to establish, or stabilize, the regular longitudinal spacing. This may have functional as well as mechanical implications, since the astrocytes form perinodal associations and their pattern is one which strongly clusters the nodes of Ranvier.

Factors determining the morphology and distribution of astrocytes in the cat retina: a 'contact-spacing' model of astrocyte interaction

The retina provides a valuable opportunity to examine the interaction of astrocytes with neurones and vasculature, in adult tissue and in vivo. We have studied astrocytes in cat retina to delineate the interactions that determine their morphology and distribution. Their morphology varied with their interaction with surrounding cells, from a classic stellate shape to an elongated bipolar form associated with axon bundles. Evidence is presented that the distribution of astrocytes across the retina is determined by their morphology and by a previously unrecognised interaction between astrocytes, which we term 'contact-spacing,' in which astrocytes maintain contact with their neighbours through their processes, but keep their somas apart. Evidence is also presented that astrocytes are not influenced in their distribution by surrounding neurones, and the influence of developmental mechanisms is identified. These observations are summarised in a contact-spacing model of astrocyte distribution, and four predictions of the model are tested. The concentration of astrocytes along axon bundles dispersed when the axons degenerate but not when vessels were prevented from forming. Further, when both axons and vessels were eliminated, the concentrations of astrocytes dispersed and they became stellate in form. Finally, in the retina of the rat, in which astrocytes show no affinity for axons, the distribution of astrocytes is essentially uniform. We suggest that the contact-spacing interaction among astrocytes provides the anatomical basis of a functional glial network extending across the retina and throughout the central nervous system.

Division of astroblasts and oligodendroblasts in postnatal rodent brain: evidence for separate astrocyte and oligodendrocyte lineages

What precursor cells are the source of the macroglia generated during postnatal development? In order to answer this question, we studied the expression of glial specific antigens in proliferating neuroglia in postnatal rodent brain and optic nerve. Immunocytochemistry using antibodies to oligodendrocyte (OL) specific markers (sulfatide and galactocerebroside) and an astrocyte (AS) specific marker (glial fibrillary acidic protein) was combined with thymidine autoradiography. During the first week of postnatal development when most ASs are being generated, one third to one half of the proliferating cells in the optic system are positive for glial fibrillary acidic protein after a 1 h injection of thymidine (Skoff, Dev. Biol., 139:149-168, 1990). During the second postnatal week when OLs are being generated, 30 to 100% of the proliferating cells in presumptive white matter tracts are sulfatide positive and at least 10% are galactocerebroside positive. This finding demonstrates that ASs and OLs divide during postnatal development. These results confirm previous electron microscopic autoradiographic studies showing that the vast majority of proliferating cells in postnatal rat optic nerve have the morphologic characteristics of differentiating ASs or OLs (Skoff, J. Comp. Neurol., 169:291-312, 1976). Since proliferating ASs (astroblasts) and OLs (oligodendroblasts) constitute the majority of the dividing cells at the time that ASs and OLs are being generated, these glioblasts must be the major source for the macroglia generated postnatally. The findings strongly suggest that separate lineages exist for ASs and OLs during postnatal development. There is no compelling in vivo evidence for a bipotential progenitor cell that generates the majority of OLs and certain ASs in postnatal rodent brain. There may, of course, be distinct lineages for the subtypes of ASs and possibly even for subtypes of OLs. We review the concepts of commitment and plasticity and apply these terms to glial differentiation. In situ, the presence of oligodendroblasts and astroblasts demonstrates the COMMITMENT of proliferating cells to a specific glial lineage during normal development. Culture conditions may provide an environment that permits proliferating glial cells to vacillate in their selection of a specific lineage. This situation demonstrates developmental PLASTICITY and the ability of glia to adapt to an altered environment. Whether committed glial cells in situ can be induced to switch their lineage when normal CNS conditions are altered is an intriguing question that remains to be answered.